1. Field of the Invention
The present invention relates to a magnetic encoder for use with a rotation detecting device for detecting the number of revolutions of one of bearing components rotatable relative to each other and also to a wheel support bearing assembly utilizing such magnetic encoder. The magnetic encoder to which the present invention pertains is suitably incorporated in the rotation detecting device of a kind utilized in association with an automobile anti-skid brake system for detecting the number of revolutions of at least one of front and rear wheels of an automotive vehicle.
2. Description of the Prior Art
The rotation detecting device for use in association with the anti-skid brake system for minimizing the hazard of an automotive vehicle being skidded on a road surface has hitherto been assembled in a number of structures. In general, however, the conventional rotation detecting device of the kind referred to above includes a serrated rotor and a detecting sensor, which are arranged spaced a distance from a sealing device used to seal the bearing assembly, functionally integrated together to define a single and independent rotation detecting device.
This conventional rotation detecting device is of a structure in which the number of revolutions of the serrated rotor mounted on a rotary shaft for rotation together therewith can be detected by the revolution detecting sensor fitted to a knuckle. The bearing assembly incorporating such rotation detecting device is protected by the sealing device, disposed independently and laterally of the rotation detecting device, from an undesirable ingress of foreign matters such as dusts and dirt and/or water.
By way of example, the Japanese Patent No. 2816783 discloses a bearing seal assembly incorporating therein a rotation detecting device, which is so designed as to contribute to reduction of the space for mounting of the rotation detecting device and to striking improvement of the detecting performance thereof. According to this patent, the bearing seal assembly includes an annular slinger and an elastic member admixed with a powdery magnetic material and bonded by vulcanization to the annular slinger, with a plurality of opposite magnetic poles defined so as to alternate circumferentially of the elastic member.
The Japanese Laid-open Patent Publication No. 6-281018 discloses a built-in coder sealing structure so designed as to reduce the axial dimension of the sealing structure, to increase the sealability between a rotating element and a stationary element and to render it to be easily mounted. According to this patent publication, a gap between the rotating element and the stationary element is sealed with a rotary disc mounted on the rotating element for rotation together therewith. A multi-pole coder is mounted on the rotary disc and built in the sealing structure. The multi-pole coder used therein is made of an elastomer added with a powdery magnetic material.
When it comes to improvement in magnetic characteristic, the larger the amount of the powdery magnetic material container in the magnetic encoder, the better the magnetic characteristic. The use of the large amount of the powdery magnetic material allows the magnetic force, generated per unit area, to be advantageously increased and the magnetic encoder can therefore be fabricated in a thin-walled structure with promising increase of the detection sensitivity. In particular, the use of the powder of rare earth magnetic material, rather than the use of the powder of ferrite magnetic material, is effective to increase the magnetic characteristic.
However, where the matrix is an elastomer or a plastomer, the amount of the powdery rare earth magnetic material to be blended cannot be increased because of the following problems found occurring during a process of mixing the matrix with the powdery rare earth magnetic material. Problems similar to those with the use of the powdery rare earth magnetic material equally apply to that of the powdery ferrite magnetic material.                (a) If the amount of the powdery magnetic material to be blended is large, a processing machine (particularly a kneading machine) is apt to be damaged considerably. The running torque tends to be so large as to impose an excessive load on the processing machine and/or, as a result of contact with the powdery magnetic material that is generally hard and tough, rolls used in the processing machine are susceptible to wear, accompanied by considerable damage.        (b) By the effect of heat evolved during the processing (blending and kneading), the powdery magnetic material is susceptible to oxidization, resulting in eventual degradation of the magnetic characteristic.        
Also, when the conventional magnetic encoder made of an elastomer or a plastomer is applied in the rotation detecting device incorporated in the wheel support bearing assembly, particulate matters such as sand particles or grit tend to be trapped in between the magnetic encoder and the sensor confronting the magnetic encoder, resulting in damage to the surface of the magnetic encoder.
In order to alleviate the above discussed problems, the applicant of the present invention has suggested, in the Japanese Laid-open Patent Publication No. 2004-037441, a magnetic encoder made of a sintered element formed by molding and sintering a powdery mixture containing a powdery magnetic material and a powdery non-magnetic material. The use of the sintered element makes it possible to increase the amount of the powdery magnetic material to be mixed and also to provide a thin-walled structure, along with increase of the resistance to wear. Also, with the magnetic encoder made of the sintered element, mixing and blending of powdery materials with each other, that is, dry blend is possible. In the case of the dry blend, the problems (a) and (b) discussed above can be remarkably alleviated.
However, it has been found that even in the magnetic encoder made of the sintered element, molding would be impossible if the amount of the powdery magnetic material is too large.